The field of this invention relates to ion exchange apparatus systems utilizing moving folded beds for loading and stripping of the resin. More particularly, this invention relates to improvements in the folded moving bed ion exchange system described in U.S. Pat. No. 4,228,001, and/or in the system disclosed in U.S. Pat. No. 4,280,904. It is believed that the systems described in these references constitute the most pertinent prior art with respect to the subject matter described in this application.
Various arrangements for folded moving bed ion exchange systems have heretofore been proposed such as those shown in U.S. Pat. Nos. 2,815,322, 3,492,092, 3,579,322, and 3,775,088. However such systems are believed to have been used commercially only for limited and specialized purposes, principally for the ion exchange treatment of boiler water. See Higgins, et al., Chem. Eng. Prog. Symposium, Ser. 50, No. 14, 87-92 (1954); and Higgins, Indust. and Engr. Chem., Vol. 53, 635-37 (1961). The assignee of the present application, American Petro-Mart, Inc., of Bartow, Florida, has been conducting an intensive pilot plant investigation of a folded moving bed ion exchange system for removal of magnesium and calcium from phosphate rockderived phosphoric acid, and for other purposes, including uranium recovery. The above-cited U.S. Pat. No. 4,228,001 and U.S. Pat. No. 4,280,904 are based on this research. This application is directed to apparatus and method improvements developed since the filing of the cited prior patents.
Since moving folded bed ion exchange systems provide a continuous resin transfer circuit between the resin loading column and the stripping or regeneration column, there is a problem in maintaining separation of the feed and product solutions of the loading column from the regenerating solutions of the regeneration column. For example, when the system is being operated for the removal of magnesium and calcium from phosphoric acid, sulfuric acid can be used advantageously as the regenerating solution, but after regeneration the resin may be returned to the loading column with residual sulfuric acid, thereby contaminating the phosphoric acid solution in the loading column and resulting in a loss of sulfuric acid. This problem is accentuated by the fact that the return of regenerated resin is to the bottom of the loading column which is the point of withdrawal of the treated feed so that any residual acid from the regenerating column tends to mix with and contaminate the product of the loading column such as the phosphoric acid from which the magnesium and calcium has been removed.
Similarily, where loaded resin is removed from the loading column for regeneration together with the solution of the loading column, such as the incoming raw phosphoric acid, a substantial loss of the feed can occur. For example, with the system of U.S. Pat. No. 4,228,001, raw feed under pump pressure is used as the transport medium for the loaded resin, the resin being removed at the top of the loading column adjacent the point of entry of the feed. Although the phosphoric acid transferred with the resin can be at least partially recovered and recycled, it is very desirable to minimize the transfer of feed liquid with the loaded resin. Also, for the reasons already indicated, it is desirable to further reduce the transfer of regenerating solution with the regenerated resin returned to the loading column.